Mild catalytic steam gasification process

ABSTRACT

The present invention provides an improved alkali metal catalyzed steam gasification process that utilizes a CO 2  trap material and/or a mineral binder material within the gasifier. The process optimally achieves over 90% carbon conversion with over 80% yield of methane. The raw gas product can be used directly as fuel. The catalyst can be recovered from the solid purge and recycled to the gasifier and/or the CO 2  trap can be regenerated and recycled to the gasifier.

This application claims priority under 35 U.S.C. 119(e) to provisionalapplication 60/695,994 which is hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to low temperature catalytic gasificationof carbonaceous material. More particularly, the present inventionrelates to an improved process for gasifying carbonaceous material thatachieves high carbon conversion to methane at mild temperatures.

BACKGROUND—DESCRIPTION OF RELATED ART

The world-wide availability of petroleum is predicted to peak and thendecline rapidly. Rapid economic, technological and industrial growth ofpopulous countries such as China and India serves to increase thisdemand, making the need for alternative sources of energy even moresevere. To meet this growing demand it has been suggested to convertcoal into more useful and transportable forms. One such technique is togasify coal into combustible gases. A coal gasification process forproducing pipeline grade fuel, such as methane, would be especiallydesirable because of the existing infrastructure adapted to transportmethane as natural gas.

In typical coal gasification systems, coal or other carbonaceousmaterials and steam are reacted with oxygen (or air) to produce asyngas, comprised primarily of hydrogen and carbon monoxide. Commercial,non-catalyzed, coal gasification systems and designs face a number ofeconomic and technical challenges. These processes are expensive tooperate since, in order to drive the endothermic non-catalyticgasification of carbonaceous materials, they utilize severe temperatures(2400 to 2600° F.) and can consume high levels of oxygen. Slagging andcorrosion also can present operating and maintenance issues which reduceeconomic viability and increase product cost.

A current concept of an Integrated Gasification Combined Cycle (IGCC)system incorporates a non-catalyzed coal gasification system to producesyngas as an intermediate and burns the syngas to produce electricity.The capital cost of an IGCC system is estimated to range from about$1,250 to $1,400 per KW, depending upon the design and processintegration. One way to reduce the cost significantly would be todevelop a process that enables one to gasify coal at lower temperatureand without added oxygen. Toward this end, it is useful to consider thethermodynamics of gasifying coal.

The gasification of coal and similar materials generally involves thefollowing reactions:C+H₂O→CO+H₂ (Endothermic)  (1)C+2H₂→CH₄ (Exothermic)  (2)C+CO₂→2CO (Endothermic)  (3)CO+H₂O≡CO₂+H₂ (Exothermic)  (4)The reaction kinetics during conventional (i.e. thermal) gasificationgenerally produce only small amounts of methane. Directhydrogenation/gasification of carbon such as depicted by equation (2)above is very slow compared to the endothermic reactions of steam andcarbon dioxide with carbon, as depicted in equations (1) and (3). Thegasification of coal and similar materials thus normally produces asynthesis gas composed primarily of hydrogen and carbon monoxide.

Addition of alkali metal catalysts enables steam gasification to proceedat lower temperatures and can enhance the production of methane throughthe following exothermic reactions:2CO+2H₂→CO₂+CH₄ (Exothermic)  (5)CO+3H₂→H₂O+CH₄ (Exothermic)  (6)CO₂+4H₂→2H₂O+CH₄ (Exothermic)  (7)

One such catalytic steam gasification process is disclosed in U.S. Pat.No. 4,094,650 to Koh et al. (“the '650 process”). The preferredtemperature and pressure ranges disclosed therein are around 1300° F.(700° C.) and 500 psia (34 atm). Potassium carbonate is disclosed as apreferred catalyst. Though the temperature is lower than innon-catalyzed gasification, the main raw products are still H₂ and CO.In order to suppress the formation of H₂ and CO, and drive the carbonconversion to methane, the '650 process teaches recycling the H₂ and COfrom the raw product. A catalyst makeup stream is also required in the'650 process because, at the temperatures therein, the alkali metalcatalyst can volatilize and/or react with ash constituents of the coalcausing a substantial decrease in catalyst activity.

Various combinations of compounds have been investigated to find lessexpensive coal gasification catalysts. For example, U.S. Pat. No.4,336,034 to Lang et al. discloses that at catalyst loadings up to 12%by weight, the relatively inexpensive combination of K₂SO₄ and calciumcompounds such as CaO, Ca(OH)₂, or CaCO₃ can provide gasification ratescomparable to relatively expensive K₂CO₃. The '034 patent reports betterperformance for mixtures having a K/Ca ratio of 2.0 (i.e., 1/3 calcium)than for mixtures with more calcium. Lang reports the use of smallamounts of calcium to enhance the activity of a relatively poor catalystsuch as K₂SO₄. There is no suggestion in Lang that higher quantities ofcalcium can influence the catalytic activity of potassium hydroxide orpotassium carbonate, or that calcium salts can be used to enhanceproduct yield, change the reaction kinetics, or enable gasification toproceed at lower operating temperatures. There is also no suggestionthat the presence of calcium can improve catalyst recovery.

Other modifications have been proposed to attain more complete carbonconversion in a catalytic coal gasification process, examples being U.S.Pat. No. 4,558,027 to McKee et al. which discloses using eutectic alkalicatalyst mixtures, and U.S. Pat. Nos. 4,077,778 and 6,955,695 to Nahaswhich disclose, respectively, using two reactors, or a two-stagereactor. These processes, like that of the '650 patent, report recyclingsubstantial quantities of H₂ and CO from the raw product gases to thegasifier to maximize the production of methane.

Thermodynamically, methane generation is favored at mild temperaturesbelow about 540° C. and high pressures, but catalytic coal gasificationprocesses typically operate hotter, i.e., at temperatures between about700° C. to about 820° C., because the gasification rate, and yield, arelow in conventional catalytic coal gasification processes at lowertemperatures.

Mild temperature coal gasification can achieve higher direct conversionof carbon to methane and can reduce or avoid catalyst losses which canoccur at higher temperatures due to binding with mineral matter in thecarbonaceous feed or volatilization. Mild temperature coal gasificationcan also minimize the conversion of coal to significantly less reactivechar. However, catalysts have not heretofore been identified that cancatalyze mild temperature gasification at acceptably high reactionrates.

Several metals (other than alkali metals) have been identified that cancatalyze steam/coal gasification, but have not shown promise for mildtemperature gasification. Transition metals such as iron or nickel cancatalyze coal gasification, but are subject to being deactivatedrapidly, after only about 10 or 15% carbon conversion. (D. Tandon, LowTemperature and Elevated Pressure Steam Gasification of Illinois Coal(1996) (Ph.D. dissertation, Southern Illinois University atCarbondale)). Research has found that unsupported Raney Ni can beseverely deactivated by H₂S, possibly due to the formation of NiAl₂S₄ onthe surface of the catalyst, but can be less affected when supported byZrO₂ and Al₂O₃.

Catalytic metals, in combination, can be less vulnerable to deactivationthan single-metal catalysts. For example, eutectic catalyst mixtures canmaintain catalytic activity longer than one constituent of the mixture.Similarly, Tandon reported that potassium combined with nickel or ironas a steam/graphite gasification catalyst can remain active longer thaniron or nickel alone. It is possible that highly dispersed alkali metalsalts can provide a reducing atmosphere for transition metal salts andthus sustain their catalytic activity.

A catalytic effect from highly dispersed calcium has also been observed.For example, the article by Yasuo Ohtsuka and Kenji Asami, “Highlyactive catalysts from inexpensive raw materials for coal gasification”,Catalysis Today 39:111 (1997) reports that calcium salts, such as CaCO₃or Ca(OH)₂, that have been “kneaded” with coal particles, can promotesteam gasification of lignite at about 550° C., but are reportedly noteffective with low-oxygen containing higher rank coals.

CaO or lime can also be used with coal conversion processes to absorbCO₂. For example, U.S. Pat. No. 4,747,938 to Khan, which is directed tocoal pyrolysis at about 550° C., discloses that using particulate CaO atup to 25 wt % loading can yield a product stream with less H₂S and CO₂.Neither the Khan nor the Ohtsuka and Asami processes utilize alkalicatalysts.

Though coal gasification catalysis has been extensively researched, itis still not completely understood. Without intending to limit thisinvention to any particular theory, it is believed that transitionmetals that can catalyze coal gasification are those which can oscillatebetween two oxidation states and participate in oxidation-reductioncycles on the carbon surface, and that gasification with alkali metalcatalysts involves the alkali metals donating electrons to the carbonlattice, or forming alkali/carbon complexes, thereby increasing thenumber of active CO complexes on the carbon surface. It is also believedthat combinations of such catalysts exhibit sustained activity becausedifferent types of active sites on the carbon surface can be activatedby different catalytic moieties, making more reaction sites availableand reducing the impact of the deactivation of any particular type ofreaction site or reaction mechanism.

It is further believed that transition metals and alkali metals arecatalytically inactive when they are oxidized, and that they can beoxidized by components of the gasification environment such as H₂O, CO₂,CO and H₂S. The alkali metal catalysts can also become inactive orineffective by volatilizing and/or binding with mineral constituents ofcoal.

It would be highly desirable to develop a catalytic coal gasificationprocess that could sustain high reactivity with high carbon conversion,and even more desirable to develop a catalytic process capable of highcarbon conversion to methane without recycling from the raw product (orfeeding) a substantial H₂ and CO stream. It would be further desirableif such a process could operate at mild temperatures where catalystlosses by vaporization or deactivation by interaction with mineralconstituents of the carbonaceous feed could be minimized. These andother objects are the subject of the process disclosed herein.

SUMMARY OF THE INVENTION

It has been found that using calcium salts to remove or “trap” carbondioxide and other oxidizing agents from a catalytic coal gasificationenvironment can shift the kinetics towards greater carbon conversion tomethane, and can also drive the conversion of CO to CO₂ such that theprocess can yield a dry raw gaseous product comprised mainly of H₂ andCH₄ and substantially free of carbon oxides. The overall coal/carbonconversion can be at least 50% but conversions greater than 95% are alsoobtainable. The process disclosed herein can directly produce a dry rawgaseous product comprised of about 40% methane or more, by volume,without the need for substantial recycling or feeding H₂ and CO to theenvironment. Advantageously, the dry raw gaseous product can be used asa fuel without further enrichment, and can provide pipeline qualitymethane with little additional treatment.

Calcium salts and other compounds can react with CO₂ and H₂S and formsolids which can be withdrawn in a solid purge, thereby eliminating orgreatly reducing the need to treat the raw gaseous product for acid gasremoval. According to the present invention, calcium salts can also bindwith, and render inert or relatively inert, mineral constituents of thecarbonaceous feed so the alkali metal salt catalysts can remain activelonger. By preventing such minerals from reacting with and deactivatingthe alkali metal catalysts, greater catalyst recovery from the solidpurge can be achieved and catalyst losses can be reduced. The processcan allow for up to ˜90% catalyst recovery.

While the invention is not limited to any theory, it is believed thatCO₂ in the gasifier causes the catalyst to deactivate, so that byeliminating the CO₂, high catalytic activity can be sustained and morecomplete conversion can be achieved. In addition, removal of CO₂ fromthe gas phase can substantially alter the ratio of hydroxide tocarbonate forms of the catalyst. Eliminating CO₂ effectively increasesthe activity of the catalyst and enables a high rate of gasification tooccur at mild operating temperatures. At mild temperatures, the kineticsfavor greater direct conversion of coal (or other carbonaceousmaterials) to methane, and the coal, which can convert to less reactivechar at conventional catalytic coal gasification temperatures, canremain more reactive. Mild temperature operation can also reducecatalyst losses and corrosion of system components caused byvolatilization of the catalyst and hazardous trace elements in thecarbonaceous feed.

The catalytic gasification processes of the present invention can alsobe simpler and less costly to build and operate than known priorprocesses, and can be less prone to overheating, corrosion, charbuild-up and other problems long associated with other gasificationprocesses and systems. The estimated Btu in, versus Btu out, efficiencycan be on the order of 80 to 85% overall.

In one embodiment of the invention there is provided a method for directcatalytic gasification of carbonaceous material to methane comprisingcausing a reaction of the carbonaceous material in an environmentincluding steam and an alkali metal salt catalyst at mild temperaturesin the range from about 300 to about 700° C. and a pressure from about15 to about 100 atmospheres, and removing CO₂ (and H₂O) from theproducts of the reaction in the environment so as to produce a dry rawgaseous product consisting of from about 30% to about 90% methane. Thus,the dry raw gaseous product can include at least about 40% methane, orat least about 50%, or at least about 60%, or even at least about 70%methane by volume. This embodiment can be carried out in the absence ofor without extensive added or recycled H₂ or CO.

Another embodiment provides an improved method for direct catalyticgasification of carbonaceous material to combustible gases, which can becarried out in the absence of added or recycled H₂ or CO, wherein thegasification reaction occurs at a temperature range from about 300 toabout 700° C. and a pressure from about 15 to about 100 atmospheres inan environment including steam, an alkali catalyst, and a mineral bindermaterial, and wherein said carbonaceous material includes silica and/oralumina, and other mineral constituents. The mineral binder material cancombine with at least a portion of these mineral constituents to inhibitthe silica and/or alumina, and other mineral constituents from combiningwith the alkali catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the invention will be more readily understoodby reference to the following detailed description. FIG. 1 is a generalFlow Diagram of a Mild Catalytic Coal Gasification (MCCG) Process inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

As used in this description and the accompanying claims, the followingterms shall have the meanings indicated, unless the context otherwiserequires:

The term “catalyst” refers to compositions that are introduced to theprocess to facilitate the gasification reactions. The term is not meantto be limited to the specific chemical moiety or moieties that activatethe carbon surface or otherwise actually participate in the gasificationreactions.

“Mild temperature gasification” as used herein, means steam gasificationof carbonaceous material at about 550° C. or lower.

“Syngas ” as used herein, means synthetically produced fuel gas,typically produced from standard coal gasification processes, comprisingmostly CO and H₂ by volume.

“Dry raw gaseous product” as used herein means non-steam orsubstantially non-steam products of direct catalytic steam gasification.Although steam can be a component of the raw gaseous reaction productsfrom direct catalytic steam gasification of carbonaceous materials,reference to ‘dry raw gaseous product’ herein means the gaseousproducts, other than steam, that flow from the gasification reactor andhave not been further purified.

“CO₂ trap material” as used herein can be CaO, Ca(OH)₂, dolomite,limestone, Trona, or other compounds effective for regenerativelycombining with CO₂ to form solid carbonates or bicarbonates, andcombinations thereof.

“Mineral binder material” as used herein can be a calcium salt, such asCaO, Ca(OH)₂, CaCO₃, or any other alkaline earth metal salts which canreact with and tie up silica, alumina, and other mineral constituents ofthe carbonaceous feed so as to inhibit such constituents from reactingwith and deactivating the catalyst.

The present invention provides a catalytic steam gasification processfor converting carbonaceous materials to gases substantially comprisingmethane or other combustible gases. The process can operate at mildtemperatures and produce a dry raw gaseous product that can be usedeither directly as fuel or purified to pipeline quality methane withoutthe need to remove therefrom substantial quantities of carbon monoxideor acid gases. The process can include a feed preparation zone, agasification reactor, a catalyst recovery system, and a CO₂ trapregeneration zone.

In the gasification reactor operating at between about 300° C. to about700° C. and with pressure in the range from about 10 atm to about 100atm, carbonaceous material can be reacted with oxidizing agents such assteam and/or oxygen in the presence of CO₂ trap material, and one ormore alkali metal salt catalysts, to produce predominantly methane asthe raw product gas. In preferred embodiments, the operating temperaturein the reactor is below about 550° C., and the pressure is in the rangefrom about 12 to about 40 atm. The gasification reactor can have amoving bed or a fluidized bed. Mineral binder material can also bepresent in the reactor, and can bind with silica, alumina, and othermineral constituents of the carbonaceous feed and thereby prevent orinhibit such constituents from reacting with and deactivating thecatalyst.

The feed preparation zone can include one or more mixers for combiningthe carbonaceous material, the alkali metal catalyst, the mineral bindermaterial, and the CO₂ trap material, and a feed system for introducingthe catalyst/carbon/CO₂ trap mixture to the gasification reactor as drysolids or as a liquid slurry. The feed system can be a star feeder,screw feeder, or other mechanism effective in maintaining requiredtemperature, pressure and flow rate of the materials to be introduced tothe gasification reactor.

The carbonaceous material can be coal, heavy oils, petroleum coke, otherpetroleum products, residua, or byproducts, biomass, garbage, animal,agricultural, or biological wastes and other carbonaceous wastematerials, etc., or mixtures thereof. The coal or other carbonaceousmaterial can be ground or pulverized to an average particle size ofabout 30 to 100 mesh before its delivery for use in the gasificationprocess. Such particles can be impregnated with alkali catalyst inaqueous solution and dried by known methods. The impregnated and driedparticles can be mixed with the CO₂ trap material and/or mineral bindermaterial and introduced to the gasifier as a single stream, or suchstreams can be fed separately, or in combination, as convenient.

In a preferred embodiment, however, the carbonaceous materials for usein the process can be more coarse, with an average particle size ofabout 1-2 mm. Such coarse particles can be combined and ground with anaqueous slurry of finely divided mineral binder material. The resultingpaste can be ground with alkali catalyst, dried at about 100° C. withsuperheated steam to recover a fine powder of carbonaceous material withhighly dispersed mineral binder and alkali catalyst having an averageparticle size of less than roughly 0.02 mm, pelletized to a particlesize of about 30-100 mesh, and fed to the gasification reactor. The CO₂trap material can be combined and fed with the prepared carbonaceousmaterial, or can be fed separately.

The CO₂ trap material can be CaO or Ca(OH)₂, or any other compound thatcan react with CO₂ to form solid carbonates or bicarbonates, so as toshift the kinetics in the direction of increased methane concentrationin the raw gas product. In particular embodiments the CO₂ trap materialis CaO. Sufficient CO₂ trap material can be used so as to removesubstantially all the CO₂ from the products of the reaction to yield adry raw gaseous product containing less than about 2% CO₂ by volume. Themolar ratio of CO₂ trap material to carbon in the reactor can be in therange of about 0.1:1 to about 1:1, or more particularly in the range ofabout 0.3:1 to about 0.7:1, and more particularly about 0.5:1. On aweight basis, if CaO is used as the CO₂ trap material, the CaO to carbonratio fed to the reactor can be in the range of about 0.5:1 to about4:1, or more particularly in the range of about 1:1 to about 3:1, andmore particularly about 2:1. The CO₂ trap material can be effectivewithout being highly dispersed on the carbon surface. Thus operatingconvenience can dictate whether the CO₂ trap material and thecarbonaceous feed are mixed and then fed or introduced separately to thegasifier.

The alkali catalyst can comprise any of Na₂CO₃, K₂CO₃, Rb₂CO₃, Li₂CO₃,Cs₂CO₃, KNO₃, K₂SO₄, LiOH, NaOH, KOH, or any suitable alkali metalsalts, or naturally occuring minerals containing alkali metal salts suchas Trona, or mixtures thereof. The catalyst can be a single compound ora combination of alkali metal salts, which can be binary or ternary saltmixtures. The alkali catalyst loading can be from 1 to 50 weight percentbased on the carbonaceous feed on a dry, ash-free basis. Preferably, thealkali loading is in the range of about 1 to 30 wt %. The alkalicatalyst can be effective without the presence of any fluorinatedcompounds.

The alkali metal salt catalyst can comprise a eutectic salt mixture ofLi₂CO₃, Na₂CO₃, K₂CO₃, Rb₂CO₃, and Cs₂CO₃ or mixtures thereof. In oneembodiment, the eutectic salt mixture can be a binary salt mixture ofabout 29% Na₂CO₃ and about 71% K₂CO₃, mole percent. In other embodimentsthe eutectic salt mixture can be a ternary composition of about 43.5%Li₂CO₃, 31.5% Na₂CO₃ and 25% K₂CO₃, mole percent, or a ternary saltmixture of about 39% Li₂CO₃, 38.5% Na₂CO₃ and 22.5% Rb₂CO₃, molepercent.

The mineral binder material can be a compound or a mixture of compoundsselected from the group consisting of CaO, Ca(OH)₂, CaCO₃, and otheralkaline earth metal salts. The mineral binder can be kneaded orotherwise dispersed on the carbonaceous feed particles in a feedpretreatment step before the alkali catalyst is contacted with thecarbonaceous feed. In the present invention, kneading calcium salts withthe carbonaceous feed particles can be used to help prevent mineralinteractions with the alkali metal catalyst. In other embodiments, thecarbonaceous feed, the mineral binder, and alkali catalyst can be mixedtogether simultaneously by conventional methods. In still furtherembodiments, the mineral binder material can be fed separately to thegasifier and/or mineral binder material can form in the gasifier,wherein such mineral binder material (e.g., CaCO₃) can react withsilica, alumina, and other mineral constituents present in thecarbonaceous feed and prevent or inhibit some alkali catalyst loss anddeactivation.

The mineral binder can combine with at least a portion of any reactivemineral constituents in the carbonaceous feed such as aluminum andsilicon constituents, and thereby prevent or inhibit such reactivemineral constituents from reacting with the alkali catalysts. Themineral binder material can thus be effective at stoichiometricquantities about equal to that of the reactive mineral constituents inthe carbonaceous feed. Thus, for example, if the carbonaceous feedmaterial is Illinois #6 coal which contains on a dry basis about 10 to11 wt % ash of which silica comprises about 51 wt % and aluminacomprises about 18 wt %, then 7.1 tons of CaCO₃ or the equivalent amountof another mineral (e.g., about 4.0 tons of CaO) would be enough toreact with all the silica and alumina in 100 tons of Illinois #6 coal.It may be preferable to use a higher or lower than stoichiometricamount, e.g. in the range of about 0.5 to about 1.5. Higher amounts ofmineral binder can promote more complete material binding, particularlyat higher operating temperatures. Lower amounts can be sufficient atmilder operating temperatures.

It may be desired to process carbonaceous feeds according to thisinvention promoting mineral binding or CO₂ trapping or both. Thus, forthe example of using CaO for either purpose with Illinois #6 coal, topromote only mineral binding, the amount of CaO utilized can be in therange of about 2 to 6 wt % and is preferably highly dispersed with thefeed; whereas to promote CO₂ trapping, higher amounts in the range of 50to 200 wt %, which need not be highly dispersed with the feed can beutilized. It is expected that substantial amounts in the range of atleast 60% to about 90% of the CO₂ trap material can be recovered in theCO₂ trap regenerator and recycled within the process, such that theamount of fresh CO₂ trap material can be about 5 to 80 wt % CaO. Topromote both mineral binding and CO₂ trapping, the feed can includeabout 5% CaO highly dispersed within the feed and the balance as aseparate stream.

The reactor is designed so that a solid purge can be periodically orcontinuously withdrawn. The CO₂ trap material reacts with CO₂ in thereactor and is withdrawn in the “carbonated” form with the solid purge.If the CO₂ trap material is CaO or Ca(OH)₂, the solid purge can includeparticles of CaCO₃, as well as particles of unreacted carbon, the ash ormineral constituents of the carbonaceous feed, and some alkali catalystin various forms.

The process of the invention can include a regeneration process ofconventional design to recover and recycle active CO₂ trap material, ifdesired or necessary. If the CO₂ trap material is CaO, for example, theCO₂ trap material regenerator can be a calciner. In such case, CaCO₃particles can be separated from said withdrawn solids by passing througha coarse sieve, or by elutriation of fine particles or other techniques,and can be directed to the calciner to recover the CaO. If necessary,the recovered CaO can be activated or its surface area increased bysteam treatment or similar treatment, during or after calcination andprior to recycling. The regenerated CaO recycled to the gasifier canconstitute as much as about 90% of the calcium value withdrawn in thesolid purge. The calcined off-gas, mostly CO₂ and possibly some CaCO₃,CaS and CaSO₄, as well as H₂S and possibly SO₂ and O₂, can besequestered or otherwise properly disposed.

The solid purge fraction that passes through the sieve can includesoluble alkali metal salts, and can also include insoluble alkali and/orcalcium aluminosilicates. These can be treated in a catalyst recoverysystem for recovery and recycle of the catalyst. The catalyst recoverysystem can comprise a water wash system and optionally can comprise alime digestion system. In one embodiment, the hot carbon/ash particlescan be contacted with water and soluble catalyst constituents of theparticles can dissolve into solution. If the particles contain smallamounts of alkali aluminosilicates, then the water contacting step canbe sufficient to accomplish essentially complete catalyst recovery. Ifthe washed solids contain appreciable amounts of insoluble alkalicomponents, the washed solids can be digested in an alkaline solution orslurry to recover insoluble alkali moieties. The washed solids cancontain sufficient calcium or other alkaline compounds such that littleor no additional lime or other alkaline solution is necessary fordigestion.

The partial pressure and/or concentration of steam can be monitored andcontrolled to maximize conversion rates and maximize overall conversionto methane or other desired gaseous product such as syngas. In someembodiments, causing the reaction includes maintaining a molar ratio ofsteam to carbon in the range of about 1.5 to 3 and/or controllingpartial pressure of the steam by addition of a non-reactive gas to thegasification environment.

The catalytic steam gasification process can produce a dry raw gaseousproduct that includes at least about 40% methane and can include atleast about 50%, or at least about 60%, or even at least about 70% orhigher, methane by volume, without the need for H₂ and CO recycling, orextensive recycling, and without the need for separate stage water-gasshift reactions. Other embodiments produce a dry raw gaseous productthat can include about 80% methane or higher.

The overall direct carbon conversion of carbonaceous material to methanecan be at least about 50%, more particularly at least about 65%, stillmore particularly at least about 80%, and still more particularly atleast about 90%. In still more particular embodiments, the carbonconversion of the carbonaceous material can be at least about 50% or atleast about 65% at less than about 550° C.

The invention and specific embodiments are described more fully in thefollowing examples:

EXAMPLE 1 Low Temperature Steam Gasification Results

Steam gasification of Illinois #6 coal was studied at elevated pressuresand low temperatures. In the absence of a catalyst at 500° C. andelevated pressure (500-1000 psig—i.e. ˜34 to 68 atm), no coal conversionwas observed. When the temperature was increased to 700° C., asignificant amount of conversion was observed. Apparently the lowertemperature is insufficient to overcome the activation energy barrier.Gas analyses at 700° C. showed no or substantially no methane formationfor de-mineralized coal samples. A small amount of methane was detectedfor the raw coal gasification. These observations are in agreement withthe findings that significant amounts of methane cannot be generated inthe absence of catalysts, and that minerals in coal can contribute tocatalysis.

The catalytic effects of iron, nickel and potassium in steamgasification were also studied. In the presence of these catalysts asubstantial amount of Illinois #6 coal was gasified at 500° C. Withsingle catalysts at about 10 wt % loading, almost 30-35 wt % coal wasgasified and most was gasified in the first 5 minutes. Methane andcarbon dioxide were the main product gases, with little to no carbonmonoxide produced. Thus, at low temperatures and elevated pressures theequilibrium is shifted to methane formation and syngas formation isminimized. Higher coal conversion and methane formation were observed at1000 psig (˜68 atm) as expected. Iron- and nickel-catalyzed reactionswere reactive for about 15 minutes, after which a sharp drop inreactivity was observed. Overall, higher conversions were obtained forde-mineralized coal samples but the methane concentrations were slightlyhigher for the raw coal gasification.

When a potassium salt was used with either iron or nickel salts as acatalyst for raw coal gasification, synergistic effects were observed.At 500° C. and 500 psig (˜34 atm) a potassium/iron salt catalyst system(5 wt % each) resulted in 42 wt % carbon conversion. The conversion wentup to 53 wt % when the catalyst loading was increased to 10 wt % each.The gas analyses showed that with this catalyst, a combination ofmethane and hydrogen production was favored. A nickel/potassium mixture5 wt % each did not show significant synergistic effects (39%conversion), which may be attributed to mineral interactions with thesesalts. At 10 wt % each, however, a conversion of 55 wt % was achieved.When the pressure was increased from 500 (˜34 atm) to 1000 psig (˜68atm) the conversion increased to 58 wt %. The gas analyses for thepotassium/nickel catalyst were comparable to the potassium/iron systemunder similar conditions.

These conversions indicate that alkali metal salts compliment transitionmetal salts in that they keep them active for longer reaction times. Theactive catalyst state may actually contain three metals (two outsidecatalysts and one from the mineral in coal). Studies also indicated thatat 500° C. and in the pressure range of 500 to 1000 psig, sodium saltswere more effective than potassium salts or any transitionmetal—potassium salt mixture. Conversions as high as 70% were obtainedwith Illinois coal. Transition metal—sodium salt mixtures were notinvestigated.

The results indicate that coal can be gasified at low temperatures andelevated pressures to produce methane, and that lower temperatures helpto minimize syngas formation.

EXAMPLE 2 MCCG in Accordance with an Embodiment of the Invention

A process flow diagram for the envisioned low temperature steamgasification process, mild catalytic coal gasification (MCCG), is shownin FIG. 1. Among the advantages for this process is, as discussed above,that it is a simple process. Particulate coal or other carbonaceousmaterial, particles of CO₂ trap material and/or mineral binder material,and an alkali metal catalyst solution, can be combined and mixed inmixer 100 to form a feed stream and fed to one or more lock hoppersshown generally as lock hopper 200. Said particulate streams can be fedseparately to mixer 100 or combined (not shown) before being fed tomixer 100. From lock hopper 200, the feed stream can be fed to gasifier300 by a screw feeder 250, which alternatively can be a star feeder, ora mechanism that feeds the carbonaceous material as a liquid slurry, orany other feed mechanism known in the art which allows carbonaceousmaterial to be fed to a gasifier at a rate, temperature and pressurenecessary to achieve the desired gasification result.

Gasifier 300 can be operated in a fluid bed 400A or a moving bed 400Bmode. Advantages of fluid bed mode 400A include ease of design and easytar control. One disadvantage of the fluid bed is that fresh feedparticles of coal and the CO₂ trap material may be removed withconverted residue (solid purge). Also the steam concentration in theoutlet gas will be higher than in the moving bed. In contrast, themoving bed mode is more complex because solid recycle is needed to movepartially gasified coal to the top of the bed to prevent tar fromleaving the reactor with the product gas. Still, an advantage of themoving bed is that the steam concentration in the outlet gas will besubstantially reduced and attrition of the CO₂ trap material isminimized. This mode also maximizes coal conversion.

When CaO, Ca(OH)₂, CaCO₃, or another alkaline earth metal salt ispresent in gasifier 300, such compounds can react with and tie upminerals in the coal or other carbonaceous material, preventing orinhibiting the minerals from reacting with the alkali metal saltcatalysts so the alkali metal salt catalysts will remain active longer,increasing the carbon conversion efficiency and carbon conversion rateand improving catalytic recovery. For example, such compounds can reactwith alumina, silica, or other mineral constituents of the coal. Thecoal or other carbonaceous feed can also be pretreated with CaO,Ca(OH)₂, CaCO₃, or other alkaline earth metal salts to tie up theminerals/ash in the coal.

Gasifier 300 is operated at about 550° C. or less and at an operatingpressure of less than about 1000 psig (68 atm). CaO, Ca(OH)₂, or othercompounds effective for regeneratively combining with CO₂ can be used asa trap for CO₂ and sulfur gases. This will enhance catalytic activity bydriving the reaction forward and will also enhance production of methaneby shifting the reaction kinetics toward increased production ofmethane.

Steam is fed to the bottom of gasifier 300. It can be beneficial to adda small quantity of O₂/air to the steam to activate the catalyst. Insuch embodiments, between about 0.1% to 3% oxygen or air is added to thesteam to provide oxidized sites on the coal surface and providecomplexes where catalyst can interact with the coal to produce highergasification rates and carbon conversion. Product gases will leave thetop of gasifier 300 and pass through a condenser 500 to remove steam.The condensed water can be used within the catalyst recovery system 600.The product gases, mostly CH₄, with lesser amounts of H₂ and NH₃ can bediverted for separation (not shown) using traditional methods, asneeded. Gas separation will be dependent on target product end use. Ifdesired, syngas is produced by lowering pressure and reducing CaO feed(or other CO₂ trap) to control the H₂/CO ratio.

Spent residue leaves the bottom of reactor/gasifier 300 and is separated(700) by a screen or other device to separate the larger sized CaCO₃particles, which form when CaO or Ca(OH)₂ is used as the CO₂ trapmaterial. The smaller sized residue is fed to extractor/catalystrecovery system, shown generally as 600, where the catalyst isdissolved, concentrated (if necessary) and recycled. Residue fromextractor 600 then goes to waste, perhaps landfill, or for by-productutilization after determination of hazard waste potential. The calciumcarbonate is calcined in calciner 800 and recycled to mixer 100. Thecalcined off gas, (mostly CO₂ and possibly some particulate CaCO₃, CaSand CaSO₄ as well as H₂S and possibly SO₂ and O₂) is ready forsequestration if the system is operated under pressure.

EXAMPLE 3 MCCG in Accordance with Another Embodiment

In other particular embodiments, coal or other carbonaceous material; aCO₂ trap material such as CaO or Ca(OH)₂ particles; and an alkali metalcatalyst solution, are mixed in mixer 100, fed to lock hopper 200, andfed to gasifier 300 as described above. Mixer 100 can comprise animpeller and means to heat the contents such that the carbonaceousparticles can become impregnated with alkali catalyst therein.

Gasifier 300 can be operated in a fluid bed 400A or a moving bed 400Bmode, as described, and is operated at a temperature between about 300°C. about 700° C. and a pressure from about 12 to about 40 atm. Asdescribed in Example 2, CaO or Ca(OH)₂ can be used as a trap for CO₂ andsulfur gases, and CaO, Ca(OH)₂, CaCO₃, or other alkaline earth metalsalts can react with alumina, silica, or other mineral constituents ofthe coal.

The remainder of the process follows that described for Example 2.

EXAMPLE 4 Test Results of Steam Gasification using KOH and CaO

Carbon conversion rate for steam gasification of Powder River Basin coal(PRB) was studied in the temperature range of 500° C. and 700° C. Carbonconversion without catalyst was about 60% after 15 minutes at 700° C.and increased to about 75% after 30 minutes. With KOH catalyst, theconversion increased to about 65% and 85% respectively. Interestingly,with KOH catalyst and CaO/C loading of 1:2 molar (about 2:1 weightratio), conversion increased to about 95% irrespective of the reactiontime. Thus, with the CaO trap material, the coal conversion isessentially complete in just about 15 minutes, showing that agasification reactor for these conditions can be designed for a shortresidence time and achieve good conversion.

The effect of temperature can be shown by comparing the conversion after30 minutes at 650° C., 600° C., and 550° C. The uncatalyzed conversionwas about 70% at 650° C., and decreased to about 50% at lowertemperatures. With KOH catalyst, the conversion was about 80% at 650° C.and decreased to about 65% and about 60% at 600° C. and 550° C.respectively. With KOH and CaO (loaded at CaO/C of 1:2 molar as above),the conversion at 650° C. was nearly 100% and decreased only slightly atthe lower temperatures to about 90%. (The conversion at 650° C. wasbetter than that observed at 700° C., which was about 95%.)

The conversion at 500° C. after just 20 minutes, using the same KOH andCaO loading, was at least 90%, and increased slightly after 50 or 60minutes. This demonstrates that the CO₂ trap enables the use of lowergasification temperatures (where methane formation is favored) and smallresidence times without unduly sacrificing conversion.

Carbon conversion rate for steam gasification of petroleum coke wasstudied at 700° C. and 650° C. Carbon conversion without catalyst at700° C. was about 35% after 15 minutes and only increased to about 45%after 60 minutes and about 55% after 90 minutes. With KOH catalyst, theconversion increased to about 45% after 15 minutes, and to about 55%,60%, and 80% after 30, 60, and 90 minutes respectively. With KOHcatalyst and CaO (again loaded at 1:2 molar CaO/C), conversion increasedto about 85% after 15 or 30 minutes and to about 95% after 60 or 90minutes. The corresponding conversions at 650° C. and 60 minutes, were15% for uncatalyzed petroleum coke, about 50% with KOH, and about 80%with the CO₂ trap. The increase in conversion with the CO₂ trap at 650°C. indicates that steam gasification of petroleum coke at 650° C. can beeconomically feasible.

EXAMPLE 5 Test Results of Steam Gasification using KOH, LiOH, NaOH, andCa(OH)₂

Carbon conversion for catalytic steam gasification in the temperaturerange of 500° C. and 700° C. of Powder River Basin coal (PRB) in thepresence of KOH, LiOH, NaOH, and Ca(OH)₂ was measured. The conversionwith KOH was about 90% and 85% respectively at 700° C. and 650° C., anddecreased to about 70% at 600° C. and to about 60% at 550° C. and 500°C. Surprisingly, NaOH showed significantly better performance of about80% conversion at 600° C. and 70% at 550° C., and performed about thesame as KOH at 700° C., 650° C. and 500° C. This suggests NaOH as apreferred low cost catalyst for low temperature steam gasification.

The conversion with LiOH was 5 to 10% lower than with NaOH, except at500° C. where LiOH gave about 65% conversion compared to about 60%conversion with NaOH, KOH, or Ca(OH)₂. The conversion with Ca(OH)₂ wasabout 70% at temperatures from 700° C. to 550° C., and dropped below 60%at 500° C.

EXAMPLE 6 Steam Gasification of Coal and Residua in Accordance with theInvention

In other particular embodiments, coal is mixed with an alkali metalcatalyst, and calcium salts selected from CaO, Ca(OH)₂, CaCO₃ and otheralkaline earth metal salts as described above, and then mixed withpetroleum residua. The coal/residua mixture is heated to about 400 to500° C. for about 3 to 30 minutes to disperse the catalyst, and then isgasified and further processed as described above.

Dispersing the catalyst allows for better catalyst contact, allowingtemperatures to be dropped to about 300° C. to about 550° C. Suchdispersal also provides better contact and reaction between reactivemineral components of the carbonaceous feed and such alkaline earthsalts, thereby avoiding mineral/catalyst interactions and enhancingcatalyst recovery. Dispersal also allows for more efficient sulfurremoval, reduced catalyst quantities, and enables extensive gasificationto result in only unreactive solids and minerals remaining aftergasification is complete. In particular embodiments, the coal to residuaweight ratio is in the range of about 1:1 to about 1:10.

The four feed components can be combined first into two streams, onecomprising coal and calcium salts, and the other comprising alkalicatalyst and residua; and such combined streams can then be mixedtogether and heated, as above, to about 400 to 500° C. for about 3 to 30minutes to disperse the alkali catalyst onto the coal. Thisadvantageously allows catalyst to be removed from potential poisons morequickly and leaves the coal mixture exposed to the catalyst only in adilute phase. Again, such dispersal allows more complete gasification ofthe carbon/residue mixture to gaseous product.

Alternatively, a small amount of residua can be combined with the coal,blended with the catalyst, and then blended with the balance of theresidua. The coal/residua/catalyst mixture is then introduced into areactor at the dispersing temperature described above (400 to 500° C.)for about 3 to 30 minutes as described, and the dispersed mixture isintroduced into another reactor where steam is added. Gasification isthen done with steam to produce gases (methane, ethane, propane andbutane) and light distillate C5 to C10 fraction (gasoline fraction). Inthis embodiment, catalyst remains dispersed in the liquid phase and onlya small amount is removed with unreacted material, allowing for bettercatalyst recovery and recycling, enhancing economics.

While the invention has been described in conjunction with a particularflow diagram, operating conditions and examples, various modificationsand substitutions can be made thereto without departing from the spiritand scope of the present invention. No limitation should be imposedother than those indicated by the following claims.

1. A method for catalytic gasification of carbonaceous material tocombustible gases, the method comprising: reacting carbonaceous materialand steam in the presence of an alkali catalyst at a temperature in therange of from about 300° C. to about 700° C. to form a gas comprisingCO₂, CH₄, H₂O and H₂; combining said CO₂ in said gas with a CO₂ trapmaterial; removing H₂O from said gas to form a dry raw gaseous product;wherein said CO₂ trap material is present in an amount sufficient tocombine with sufficient quantities of CO₂ to form a dry raw gaseousproduct comprising at least about 40% methane by volume.
 2. A method forcatalytic gasification of carbonaceous material to combustible gases,the method comprising: reacting carbonaceous material and steam in thepresence of an alkali catalyst at a temperature in the range of fromabout 300° C. to about 700° C. to form a gas comprising CO₂, CH₄ and H₂,wherein said carbonaceous material includes silica, alumina, and othermineral constituents; and providing a mineral binder material to combinewith at least a portion of said mineral constituents to inhibit saidmineral constituents from combining with said alkali catalyst.
 3. Amethod for catalytic gasification of carbonaceous material tocombustible gases, the method comprising: reacting carbonaceous materialand steam in the presence of an alkali catalyst at a temperature in therange of from about 300° C. to about 700° C. to form a gas comprisingCO₂, CH₄, H₂O and H₂, wherein said carbonaceous material includessilica, alumina, and other mineral constituents; providing a mineralbinder material to combine with at least a portion of said mineralconstituents to inhibit said mineral constituents from combining withsaid alkali catalyst. combining said CO₂ in said gas with a CO₂ trapmaterial; removing H₂O from said gas to form a dry raw gaseous product;wherein said CO₂ trap material is present in an amount sufficient tocombine with sufficient quantities of CO₂ to form a dry raw gaseousproduct comprising at least about 40% methane by volume.
 4. A methodaccording to claim 1, 2, or 3, wherein the temperature is in the rangefrom about 300° C. to about 550° C.
 5. A method according to claim 1, 2,or 3, wherein substantial quantities of H₂ and/or CO are not recycled oradded to the reactor.
 6. A method according to claim 1, 2, or 3, whereinthe alkali catalyst comprises one or more compounds selected from thegroup consisting of Na₂CO₃, K₂CO₃, Rb₂CO₃, Li₂CO₃, Cs₂CO₃, KNO₃, K₂SO₄,LiOH, NaOH, KOH and naturally occuring minerals containing alkali metalsalts.
 7. A method according to claim 1 or 3, wherein said CO₂ trapmaterial comprises one or more compounds selected from the groupconsisting of CaO, Ca(OH)₂, dolomite, limestone, Trona, and othercompounds effective for regeneratively combining with CO₂ to form solidcarbonates and bicarbonates.
 8. A method according to claim 7 whereinsaid CO₂ trap material comprises CaO.
 9. A method according to claim 8wherein the weight ratio of CaO to carbon in the reactor is in the rangeof about 0.5:1 to about 4:1.
 10. A method according to claim 9 whereinthe weight ratio of CaO to carbon in the reactor is about 2:1.
 11. Amethod for catalytic gasification of carbonaceous material tocombustible gases, the method comprising: reacting carbonaceous materialand steam in an environment in the presence of an alkali catalyst and aquantity of CO₂ trap material at a temperature in the range from about300° C. to about 700° C. to form a gas comprising CH₄ and H₂O and solidparticles comprising carbonated CO₂ trap material; removing H₂O fromsaid gas to form a dry raw gaseous product comprising at least about 30%methane; removing said solid particles from the environment,regenerating CO₂ trap material therefrom, and returning said regeneratedCO₂ trap material to said environment.
 12. A method according to claim11 wherein said regenerated CO₂ trap material comprises at least 50% ofsaid quantity of CO₂ trap material.
 13. A method according to claim 12wherein said regenerated CO₂ trap material comprises at least 90% ofsaid quantity of CO₂ trap material.
 14. A method according to claim 2 or3, wherein said mineral binder material comprises one or more compoundsselected from the group consisting of CaO, Ca(OH)₂, CaCO₃, and otheralkaline earth metal salts.
 15. A method according to claim 2 or 3further comprising dispersing said mineral binder material into saidcarbonaceous material prior to said reacting.
 16. A method according toclaim 14 wherein the stoichiometric ratio of said mineral bindermaterial relative to said mineral constituents of said carbonaceousmaterial is in the range of about 0.5 to about 1.5.
 17. A methodaccording to claim 14 wherein the stoichiometric ratio of said mineralbinder material relative to said mineral constituents of saidcarbonaceous material is about 1:1.
 18. A method according to claim 1, 2or 3 wherein the carbon conversion of the carbonaceous material is atleast about 50%.
 19. A method according to claim 18 wherein the carbonconversion of the carbonaceous material is at least about 65%.
 20. Amethod according to claim 19, wherein the carbon conversion of thecarbonaceous material is at least 80%.
 21. A method according to claim 1or 3 wherein the dry raw gaseous product includes at least about 50%methane by volume.
 22. A method according to claim 21 wherein the dryraw gaseous product includes at least about 60% methane by volume.
 23. Amethod according to claim 21 wherein the dry raw gaseous productincludes at least about 70% methane by volume.
 24. A method according toclaim 21 wherein the dry raw gaseous product includes at least about 80%methane by volume.
 25. A method according to claim 1, 2, or 3, furthercomprising maintaining the molar ratio of steam to carbon in the reactorwithin the range of about 1.5:1 to 3:1.
 26. A method according to claim1, 2, or 3, further comprising controlling the partial pressure of thesteam by addition of a non-reactive gas to the reactor.
 27. The methodaccording to claim 1, 2, or 3 wherein the reactor comprises a fluid bedor a moving bed.
 28. A method according to claim 6 wherein the alkalicatalyst comprises a eutectic salt mixture.
 29. A method according toclaim 28, wherein the eutectic salt mixture is a binary salt mixture.30. A method according to claim 29 wherein the binary salt mixture is29% Na₂CO₃ and 71% K₂CO₃ by mole percent.
 31. A method according toclaim 28, wherein the eutectic salt mixture is a ternary salt mixture.32. A method according to claim 31 wherein the ternary salt mixture is43.5% Li₂CO₃, 31.5% Na₂CO₃ and 25% K₂CO₃ by mole percent.
 33. A methodaccording to claim 31 wherein the ternary salt mixture is 39% Li₂CO₃,38.5% Na₂CO₃ and 22.5% Rb₂CO₃ by mole percent.
 34. A method according toclaim 4 wherein the alkali catalyst comprises NaOH, Na₂CO₃, or Trona.35. A method for catalytic gasification of carbonaceous material tocombustible gases, the method comprising: reacting carbonaceous materialand steam in the presence of an alkali catalyst at a temperature in therange of from about 300° C. to about 700° C. to form a gas comprisingCO₂, CH₄, H₂O and H₂; combining said CO₂ in said gas with a CO₂ trapmaterial; removing H₂O from said gas to form a dry raw gaseous product;wherein said CO₂ trap material is present in an amount sufficient tocombine with sufficient quantities of CO₂ so said dry raw gaseousproduct comprises less than about 2% CO₂ by volume.